An electrodeless discharge lamp is well known in the art. U.S. Pat. No. 3,872,349, discloses a typical electrodeless lamp powered by a microwave energy to generate plasma within the bulb and thereby generate visible and invisible light.
A standard electrodeless lamp uses a photocell that picks up visible light coming from the bulb and is part of the control circuit to indicate whether the bulb during starting has started or if the bulb has stopped running. For safety reasons, it is essential that the microwave energy source be turned off if the bulb stops working or if the bulb fails to start. The photocell is generally disposed outside of the microwave cavity and is directed through one of the ventilating holes such that the light radially emitted from the bulb enters the photocell, effecting the state of the photocell which can be sensed by the associated electronic circuitry.
The standard electrodeless lamps are typically used in concert with other lamps in a number of geometries, such as side by side, in rows or in a configuration where the lamps face each other. An improperly operating lamp, e.g., a lamp that fails to start or fails during operation, may not be detected by its own photocell because of stray light coming from the other lamps operating nearby. The resulting cross-interferences between the lamps prevents the prior art photocell configuration from discriminating between light coming from its own bulb and light that is coming in from another lamp.
There is, therefore, a need for a light sensing device for sensing the light output of a bulb that obviates the shortcomings of prior art devices.